If I want to select all records in a table that have not been processed yet and then update those records to reflect that they have been processed, I would do the following:

SELECT * FROM [dbo].[MyTable] WHERE [flag] IS NULL;
UPDATE [dbo].[MyTable] SET [flag] = 1 WHERE [flag] IS NULL;

How do I ensure that the UPDATE works on only the records I just selected, ie, prevent the UPDATE of any records that may have been added with [flag] = NULL that occurred AFTER my SELECT but before my UPDATE by another process? Can I wrap those two statements in a transaction? Do I have to put a lock on the table?